The object of the present invention is a thermoroll for a paper/board machine or a finishing or converting machine, the said thermoroll comprising a shell of metallic, ceramic or composite material, the shell incorporating ducts for passing a heating medium from one axial end of the shell to its opposite end.
A further object of the invention is a method for manufacturing a thermoroll for a paper/board machine or a finishing or converting machine, the said thermoroll comprising a shell of metallic, ceramic or composite material, the said shell incorporating ducts for passing a heating medium from one axial end of the shell to its opposite end.
Heatable rolls, that is, thermorolls, are commonly used in paper machines and paper finishing or converting machines, especially in calenders and multi-roll calenders, the length of the said thermorolls being as much as 10 m, their diameters being typically of the order of approximately 500-1000 mmxe2x80x94with soft calender rolls 1200-1650 mm. The heating of the rolls is usually carried out by means of a heating medium, such as steam or hot water or oil. Thermorolls are typically formed by drilling axial bores close to the outer surface of the roll shell, the diameter of the bores being typically about 25-50 mm, and through which bores the heating medium is passed from one axial end of the roll to its opposite end. There are typically several such bores, distributed evenly in the circumferential direction of the roll. The heating medium may circulate in the bore, for example, once from one end of the roll to the other, or twice or several times so that in adjacent bores, the heating medium travels in opposite directions. FIGS. 1 and 2 show a prior art thermoroll of this type, in which a shell 2 is attached to end flanges 16, 18 provided with axle journals 15, 17, in which shell are formed axial bores 3, of which there are several, distributed evenly in the circumferential direction. In the axle journal 15 is formed an axial bore 14, to which is fitted a pipe 11 extending to the opposite end flange 18. Between the outer surface of the pipe 11 and the interior surface of the axial bore 14 remains an annular slot. The heating medium is supplied to the roll 1 from the first end (the end with the axle journal 15) through a pipe 11 and passed via the radial bores 12 at the opposite end to the bores 3, and along them back to the first end, and via the radial bores 13 to the said annular slot in the axle journal 15 and from there out of the roll.
A problem associated with thermorolls provided with this type of prior art bores relates to making the axial bores by means of long hole drilling, which is relatively slow and expensive. Long hole drilling is made particularly demanding by the formation of material structure boundary surfaces in the wall construction of the shell due to the manufacturing technique. The cementite microstructure in chill cast thermorolls is brittle and susceptible to breakage due to the effect of mechanical and thermal loads. Variation in the thickness of the cementite layer may in addition cause curving of the rolls when heated. Intergranular corrosion may also occur due to paper auxiliaries. Furthermore, the current trend towards increasingly high temperatures increases the problems caused by the thermal fatigue of materials. To improve wear resistance, chilled rolls have to be coated, for example, by hard chromium plating.
Thus, one of the aims of the present invention is to achieve an improved thermoroll, where long hole drilling and other prior art disadvantages are avoided. The aim is, moreover, to achieve a roll, where good heating properties are obtained for the outer surface of the roll. To achieve this aim, it is characteristic of the thermoroll relating to the invention that the shell is made by means of casting or powder metallurgical methods, and that the ducts are formed in the matrix material of the shell which is of metal, ceramic or a composite, directly in connection with manufacture. Other preferred embodiments of the thermoroll relating to the invention are described in dependent claims 2 to 11.
Of the method relating to the invention for manufacturing a thermoroll it is, on the other hand, characteristic that the shell is made by means of casting and/or powder metallurgical methods, and that the ducts are formed in the matrix material of the shell which is of metal, ceramic or a composite, directly in connection with manufacture, without machining. Other preferred embodiments of the method relating to the invention are described in dependent claims 13 to 17.